Diffusers of active substances used to diffuse fragrances, insecticides and others to the environment may incorporate a container vessel that houses the volatile substance, provided with a porous membrane that allows the vapour of said substance but not the liquid to pass.
A number of problems related to the manufacturing and operation of this type of device can be identified, as described below.
During the filling process, the liquid substance introduced in the container may splash the area of the container on which the membrane is soldered, producing a humidity that affects the quality of the weld. This means that the membrane may move, resulting in a loss of liquid.
To prevent splashes on the welding area, two containers are used with a smaller volume than foreseen, resulting in a large air chamber inside the container. This means that the user has the impression of purchasing an incomplete product, in which part of the liquid has been used.
Another solution to avoid the problem of undesired splashes is increasing the viscosity of the products to be evaporated, using very viscous gels or liquids with the problem that these products have a considerable non-volatile residue.
Another problem related to this type of devices relates to the fact that during the evaporation of the substance a vacuum is generated inside the container, as no air enters it. Thus, the membrane is sucked in causing it to bend inwards, generating tensions that may lead to its breakage at its area of attachment to the container.
In this situation the volume of the container is reduced as the membrane deforms, so that the consumer may observe an liquid level identical or even higher than the initial level when comparing it to the altered useful volume of the container. This means that it is not possible to appreciate a gradual evolution of its consumption.
In addition, during the evaporation the liquid level changes, so that the contact surface of the membrane and the liquid is reduced, resulting in a slower evaporation rate. The reduced evaporation rate along the product lifetime implies a depreciation of its functionality, the consumer noticing this significant loss of performance.
Another problem relates to the fact that membrane systems are only permeable to volatile substances, not to the remaining components that form part of the substance such as those used to modify its viscosity, protect its colour, antioxidants, colorants and others. These products remain in the container when the volatile liquid has evaporated completely, so that it is difficult for the end consumer to appreciate when its use has finished, as an unevaporated remainder is still observed inside the container.